In design of the conventional transmission apparatus, generally there is a trade-off relationship between efficiency and linearity. However, in the transmitting apparatus, recently there is proposed a technology in which a balance between the high efficiency and the linearity can be established using polar modulation.
As to the conventional polar modulation transmission circuit, for example, Patent Literature 1 discloses a transmission circuit. FIG. 1 is a block diagram illustrating an example of a configuration of conventional transmission circuit 10 disclosed in Patent Literature 1. In FIG. 1, conventional transmission circuit 10 includes amplitude/phase extraction section 11, phase modulation section 12, amplifier 13, output terminal 14, and amplitude control section 15.
Amplitude/phase extraction section 11 extracts an amplitude signal indicating an amplitude component (for example, √(I2+Q2)) and a phase signal indicating a phase component (for example, an angle formed by a modulation symbol and an I-axis) from input data. The amplitude signal is input to amplitude control section 15. Amplitude control section 15 supplies a voltage corresponding to the amplitude signal as a power supply voltage to amplifier 13. The phase signal is input to phase modulation section 12. Phase modulation section 12 performs phase modulation of a high-frequency signal based on the input phase signal, and outputs a phase modulation signal obtained by the phase modulation. The phase modulation signal is input to amplifier 13. Amplifier 13 amplifies the phase modulation signal according to the voltage supplied from amplitude control section 15. The signal amplified by amplifier 13 is output as a transmission signal from output terminal 14. An output level of the transmission signal can be controlled by changing an output voltage of amplitude control section 15 to be supplied to amplifier 13. A method in which the amplitude signal and the phase signal are separated from the input data to perform the modulation using the amplitude signal and the phase signal is called polar modulation or polar coordinate modulation. Transmission circuit 10 that performs the method is also called a polar modulation transmission circuit (or polar coordinate modulation transmission circuit).
In the polar modulation transmission circuit, there is known a distortion generated in amplifier 13. A relationship (AM-AM characteristic) between an output power and the power supply voltage corresponding to the amplitude signal is not linear in amplifier 13. Additionally, a relationship (AM-PM characteristic) between a phase deviation of input/output and the power supply voltage of amplifier 13 is not constant. Particularly, when the AM-PM characteristic changes, a shape of a spectrum of the output signal becomes asymmetry. As a result, radio performance such as an ACLR (Adjacent Channel Lockage Power Ratio) is degraded in a transmitter.
An influence of the AM-PM characteristic on the ACLR will be described below with reference to FIGS. 2 and 3. The AM-PM characteristic means a phase characteristic that changes according to power supply voltage Vcc of amplifier 13. FIG. 2 illustrates an example of a relationship between power supply voltage Vcc of amplifier 13 and passage phase Ph, namely, the AM-PM characteristic of amplifier 13. In FIG. 2, characteristic 21 indicates the ideal AM-PM characteristic, and characteristic 22 indicates the actual AM-PM characteristic. While the ideal AM-PM characteristic is flat, passage phase Ph varies according to power supply voltage Vcc of amplifier 13 in the actual AM-PM characteristic.
FIG. 3 illustrates an example of a spectrum when amplifier 13 has the AM-PM characteristic illustrated in FIG. 2. In FIG. 3, broken line 31 indicates the spectrum of the ideal AM-PM characteristic indicated by characteristic 21 of FIG. 2, and solid line 32 indicates the spectrum of the AM-PM characteristic when passage phase Ph varies according to power supply voltage Vcc as indicated by characteristic 22 of FIG. 2. When the AM-PM characteristic is not flat, the ACLR is degraded as illustrated in FIG. 3.
As described above, in the polar modulation transmission circuit, the relationship (AM-AM characteristic) between the power supply voltage of the amplifier and the output power and the relationship (AM-PM characteristic) between the power supply voltage and the phase deviation of the input/output are compensated in order to maintain quality of the transmission signal.
A pre-distortion distortion compensation method is known as a method for compensating a generated distortion component through such compensation (for example, see Patent Literature 2). In a polar modulation transmission circuit described in Patent Literature 2, the AM-AM characteristic and the AM-PM characteristic are previously acquired and stored in a distortion compensation processing circuit, and reverse characteristics of the AM-AM characteristic and the AM-PM characteristic are added to the amplitude signal and the phase signal. By this means, it is possible to compensate the characteristic degradation caused by the distortion generated in the amplifier.